According to the differential extreme principle, if the first derivative of the profit function is equal to zero, that is, the marginal profit is equal to zero, and the marginal revenue is equal to the marginal cost, that is, the marginal profit is equal to zero, then the profit reaches the maximum. The price at this time is the best selling price.

Marginal income is the change of income caused by the change of sales volume by one unit. That is, the income from selling one unit more will increase, or the income from selling one unit less will decrease. The correlation between marginal revenue and sales volume is: in a perfectly competitive market, because the sales price of products is determined by the total market supply and total demand, the market sales price of products operated by a single enterprise is a specific constant, so the corresponding increase in revenue-marginal revenue is its sales price, that is, marginal revenue is equal to the sales price.

Marginal cost refers to the increment of total cost per unit of newly produced products (or purchased products). This concept shows that the cost per unit product is related to the total product.